Coupled modelling of hydration, calcium leaching, and sulfate attack in fly ash–blended cementitious materials

Abstract

Low-calcium fly ash is widely used in cementitious materials to improve their resistance to external sulfate attack (ESA). This study presents a meso-scale numerical framework to evaluate the durability of systems incorporating different levels of fly ash under ESA conditions. The initial phase assemblage is determined using a hydration model that accounts for various fly ash chemical compositions and replacement ratios. The degradation process is simulated by coupling chemical reactions, ion transport, calcium leaching, porosity development, and mechanical damage. A random porosity field, characterized by both statistical distribution and spatial correlation, is introduced to represent material heterogeneity. Following model validation, the framework is applied to investigate the effects of sulfate concentration, specimen size, and fly ash content on sulfate resistance. The study further investigates the interaction between sulfate attack and calcium leaching, the influence of fly ash chemical composition on expansion strain, and the relationship between mineralogical indicators and ESA performance. Results indicate that neglecting the coupling between sulfate attack and calcium leaching leads to a significant underestimation of material degradation. In addition, smaller specimen sizes, neglecting porosity evolution, and a higher coefficient of variance (CV) of the porosity field result in increased expansion. Among the hydration products, the equivalent calcium aluminate (CA) content shows a positive linear correlation with expansion. Fly ash-related chemical indicators exhibit considerable variability and limited predictive accuracy; nevertheless, lower indicator values are generally associated with improved resistance to external sulfate attack.